Particle image velocimetry of extreme ultraviolet lithography systems

ABSTRACT

A method includes irradiating a target droplet in an extreme ultraviolet light source of an extreme ultraviolet lithography tool with light from a droplet illumination module. Light reflected and/or scattered by the target droplet is detected. Particle image velocimetry is performed to monitor one or more flow parameters inside the extreme ultraviolet light source.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a Divisional of U.S. patent application Ser. No.17/340,762 filed on Jun. 7, 2021, which is a Continuation of U.S. patentapplication Ser. No. 16/579,660 filed on Sep. 23, 2019, now U.S. Pat.No. 11,029,324, which claims priority to U.S. Provisional Application62/738,394 filed on Sep. 28, 2018, the entire disclosures of each ofwhich are incorporated herein by reference.

BACKGROUND

The wavelength of radiation used for lithography in semiconductormanufacturing has decreased from ultraviolet to deep ultraviolet (DUV)and, more recently to extreme ultraviolet (EUV). Further decreases incomponent size require further improvements in resolution oflithography, which are achievable using extreme ultraviolet lithography(EUVL). EUVL employs radiation having a wavelength of about 1-100 nm.One method for producing EUV radiation is laser-produced plasma (LPP).In an LPP-based EUV source, a high-power laser beam is focused on smalldroplet targets of metal, such as tin, to form a highly ionized plasmathat emits EUV radiation with a peak maximum emission at 13.5 nm.

The collector mirror reflectance is an important factor in an EUVradiation source for an EUVL system. The reflective quality of thecollector mirror directly affects the power and wavelength of thereflected EUV light rays. A low quality collector mirror having uneventhickness, uneven surface roughness, and non-uniform reflectance oflayers in the mirror, reduces the total amount of reflected EUV lightrays and the reflected EUV light rays have a lower power and differentor a mixture of wavelengths, compared with the EUV light rays directlygenerated from the plasma. The collector mirror is subject tocontamination. For example, plasma formation during the EUV light raygeneration also generates debris, which may deposit on the reflectivesurface of the collector mirror, thereby contaminating the reflectivesurface of the collector mirror and lowering the quality of thereflected EUV light rays. Thus, EUV collector mirrors have a limitedservice life, as they tend to be fouled by accumulating tin debris,which degrades the reflectance of the collector mirror when in use.Thus, the EUV collector mirror needs to be replaced due to the debriscontamination. Each time a fouled/contaminated collector mirror isreplaced, several days of production are lost for the EUVL system,because the optics between the collector mirror, source, and scannerhave to be re-aligned. A monitoring system to determine when the EUVcollector mirror needs to be replaced is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a schematic view of an EUV lithography system with a laserproduced plasma (LPP) EUV radiation source in accordance with someembodiments of the present disclosure.

FIG. 2 is a schematic view of an EUV lithography exposure tool inaccordance with some embodiments of the present disclosure.

FIG. 3 shows a schematic view of plasma formation process throughlaser-metal interaction between a laser beam and a metal droplet inaccordance with some embodiments of the present disclosure.

FIG. 4 shows a cross-sectional view of the EUV radiation source in anoperation situation in accordance with some embodiments of the presentdisclosure.

FIG. 5A shows a schematic view of a collector mirror and relatingportions of an EUV radiation source in accordance with some embodimentsof the present disclosure.

FIG. 5B shows a detailed view of drip holes and a debris receptacle inaccordance with some embodiments of the present disclosure.

FIG. 6A shows contamination of the EUV collector mirror in the chamberof the EUVL system in accordance with some embodiments of the presentdisclosure.

FIG. 6B shows an EUV collector mirror after cleaning the surface thereofin accordance with some embodiments of the present disclosure.

FIGS. 7A and 7B show devices for illuminating and imaging tin dropletsand tin debris in an EUV radiation source in accordance with someembodiments of the present disclosure.

FIG. 8A schematically illustrates an apparatus for measuring a speed ofthe target droplet DP, the debris droplets, or the debris in a EUVradiation source, in accordance with some embodiments of the presentdisclosure.

FIG. 8B is a detailed view of an opaque barrier having slits used in theapparatus of FIG. 8A in accordance with an embodiment of the presentdisclosure.

FIG. 9A is an exemplary graph of the gas flow in an extreme ultravioletradiation source according to embodiments of the present disclosure.

FIG. 9B is an exemplary graph of the particle flow in an extremeultraviolet radiation source according to embodiments of the presentdisclosure.

FIG. 10 illustrates a flow diagram of an exemplary process forvelocimetry of droplets of an EUV lithography system according to someembodiments of the disclosure.

FIGS. 11A and 11B illustrate an apparatus for velocimetry of droplets ofdebris of an EUV lithography system and monitoring collector mirrorcontamination, according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“being made of” may mean either “comprising” or “consisting of.” In thepresent disclosure, a phrase “one of A, B and C” means “A, B and/or C”(A, B, C, A and B, A and C, B and C, or A, B and C), and does not meanone element from A, one element from B and one element from C, unlessotherwise described.

The present disclosure is generally related to extreme ultravioletlithography (EUVL) systems and methods. More particularly, it is relatedto apparatuses and methods for monitoring the contamination on acollector mirror in a laser produced plasma (LPP) EUV radiation source.The collector mirror, also referred to as an LPP collector mirror or anEUV collector mirror, is an important component of the LPP EUV radiationsource. It collects and reflects EUV radiation and contributes tooverall EUV conversion efficiency. However, it is subjected to damageand degradation due to the impact of particles, ions, radiation, anddebris deposition. In particular, tin (Sn) debris is one of thecontamination sources of the EUV collector mirror. An EUV collectormirror life time, the duration of the reflectivity decays to half ofitself, is one of the most important factors for an EUV scanner. Themajor reason for decay of the collector mirror is the residual metalcontamination (tin debris) on the collector mirror surface caused by theEUV light generation procedure.

The excitation laser heats metal (e.g., tin) target droplets in the LPPchamber to ionize the droplets to a plasma which emits the EUVradiation. During laser-metal interaction, a tin droplet may be missedby or not interact sufficiently with the laser beam, forming debris.Also, some tin leftover from the plasma formation process can becomedebris. The debris can accumulate on the surface of the EUV collectormirror, deteriorating the reflective quality of the EUV collectormirror. Monitoring the flow of the debris in the EUV radiation source isimportant to determine how the debris move and where the debris aredeposited. Parameters that are monitored and controlled in the EUVradiation source, in some embodiments, include the flow pattern of thegases, metal droplets (e.g., tin droplets), and debris in the EUVradiation source; debris propagation direction and speed; and spatialevolution of the plasma shockwave. The flow pattern of the metaldroplets and debris may be determined by observing the metal dropletsand debris particles in successive images taken from inside of the EUVradiation source and determining the velocity of the metal droplets anddebris particles. In some embodiments, the flow pattern of the gases aredetermined based on the flow pattern of metal droplets and/or debrisparticles. Monitoring the flow pattern of the metal droplets and debrisin the EUV radiation source of the EUVL system, may determine a map ofan amount of debris that are deposited on the collector mirror. Based onthe map of the amount of debris on the collector mirror, it may bedetermined when EUV collector mirror half life time is reached, when toclean the collector mirror, or when to replace the collector mirror.

A droplet illumination modules (DIM) is used to illuminate the inside ofthe EUV radiation source and a droplet detection module (DDM) is used tomeasure the parameters corresponding with the particles of the debris.The DIM directs non-ionizing light, e.g., a laser light, to the targetdroplet and the reflected and/or scattered light is detected by the DDM.The light from the DIM is “non-ionizing” and the light from the DIM isused to illuminate the metal droplets and debris inside the EUVL system.The embodiments of the present disclosure are directed to controllingdroplet illumination and detection for accurately measuring theparameters related to the metal droplets and debris inside the EUVLsystem and particularly near the collector mirror.

FIG. 1 is a schematic view of an EUV lithography system with a LPP-basedEUV radiation source, in accordance with some embodiments of the presentdisclosure. The EUV lithography system includes an EUV radiation source100 (an EUV light source) to generate EUV radiation, an exposure device200, such as a scanner, and an excitation laser source 300. As shown inFIG. 1 , in some embodiments, the EUV radiation source 100 and theexposure device 200 are installed on a main floor MF of a clean room,while the excitation laser source 300 is installed in a base floor BFlocated under the main floor. Each of the EUV radiation source 100 andthe exposure device 200 are placed over pedestal plates PP1 and PP2 viadampers DMP1 and DMP2, respectively. The EUV radiation source 100 andthe exposure device 200 are coupled to each other by a couplingmechanism, which may include a focusing unit.

The lithography system is an EUV lithography system designed to expose aresist layer by EUV light (also interchangeably referred to herein asEUV radiation). The resist layer is a material sensitive to the EUVlight. The EUV lithography system employs the EUV radiation source 100to generate EUV light, such as EUV light having a wavelength rangingbetween about 1 nm and about 100 nm. In one particular example, the EUVradiation source 100 generates an EUV light with a wavelength centeredat about 13.5 nm. In the present embodiment, the EUV radiation source100 utilizes a mechanism of laser-produced plasma (LPP) to generate theEUV radiation.

The exposure device 200 includes various reflective optical components,such as convex/concave/flat mirrors, a mask holding mechanism includinga mask stage, and wafer holding mechanism. The EUV radiation generatedby the EUV radiation source 100 is guided by the reflective opticalcomponents onto a mask secured on the mask stage. In some embodiments,the mask stage includes an electrostatic chuck (e-chuck) to secure themask. Because gas molecules absorb EUV light, the lithography system forthe EUV lithography patterning is maintained in a vacuum or a-lowpressure environment to avoid EUV intensity loss. The exposure device200 is described in more details with respect to FIG. 2 .

In the present disclosure, the terms mask, photomask, and reticle areused interchangeably. In some embodiments, the mask is a reflectivemask. In some embodiments, the mask includes a substrate with a suitablematerial, such as a low thermal expansion material or fused quartz. Invarious examples, the material includes TiO₂ doped SiO₂, or othersuitable materials with low thermal expansion. The mask includesmultiple reflective layers (ML) deposited on the substrate. The MLincludes a plurality of film pairs, such as molybdenum-silicon (Mo/Si)film pairs (e.g., a layer of molybdenum above or below a layer ofsilicon in each film pair). Alternatively, the ML may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configurable to highly reflect the EUV light. The mask mayfurther include a capping layer, such as ruthenium (Ru), disposed on theML for protection. The mask further includes an absorption layer, suchas a tantalum boron nitride (TaBN) layer, deposited over the ML. Theabsorption layer is patterned to define a layer of an integrated circuit(IC). Alternatively, another reflective layer may be deposited over theML and is patterned to define a layer of an integrated circuit, therebyforming an EUV phase shift mask.

The exposure device 200 includes a projection optics module for imagingthe pattern of the mask on to a semiconductor substrate with a resistcoated thereon secured on a substrate stage of the exposure device 200.The projection optics module generally includes reflective optics. TheEUV radiation (EUV light) directed from the mask, carrying the image ofthe pattern defined on the mask, is collected by the projection opticsmodule, thereby forming an image on the resist.

In various embodiments of the present disclosure, the semiconductorsubstrate is a semiconductor wafer, such as a silicon wafer or othertype of wafer to be patterned. The semiconductor substrate is coatedwith a resist layer sensitive to the EUV light in presently disclosedembodiments. Various components including those described above areintegrated together and are operable to perform lithography exposingprocesses. The lithography system may further include other modules orbe integrated with (or be coupled with) other modules.

As shown in FIG. 1 , the EUV radiation source 100 includes a targetdroplet generator 115 and a LPP collector mirror 110, enclosed by achamber 105. A droplet DP that does not interact goes to droplet catcher85. The target droplet generator 115 generates a plurality of targetdroplets DP, which are supplied into the chamber 105 through a nozzle117. In some embodiments, the target droplets DP are tin (Sn), lithium(Li), or an alloy of Sn and Li. In some embodiments, the target dropletsDP each have a diameter in a range from about 10 microns (μm) to about100 μm. For example, in an embodiment, the target droplets DP are tindroplets, each having a diameter of about 10 μm, about 25 μm, about 50μm, or any diameter between these values. In some embodiments, thetarget droplets DP are supplied through the nozzle 117 at a rate in arange from about 50 droplets per second (i.e., an ejection-frequency ofabout 50 Hz) to about 50,000 droplets per second (i.e., anejection-frequency of about 50 kHz). For example, in an embodiment,target droplets DP are supplied at an ejection-frequency of about 50 Hz,about 100 Hz, about 500 Hz, about 1 kHz, about 10 kHz, about 25 kHz,about 50 kHz, or any ejection-frequency between these frequencies. Thetarget droplets DP are ejected through the nozzle 117 and into a zone ofexcitation ZE at a speed in a range from about 10 meters per second(m/s) to about 100 m/s in various embodiments. For example, in anembodiment, the target droplets DP have a speed of about 10 m/s, about25 m/s, about 50 m/s, about 75 m/s, about 100 m/s, or at any speedbetween these speeds.

The excitation laser beam LR2 generated by the excitation laser source300 is a pulsed beam. The laser pulses of laser beam LR2 are generatedby the excitation laser source 300. The excitation laser source 300 mayinclude a laser generator 310, laser guide optics 320 and a focusingapparatus 330. In some embodiments, the laser generator 310 includes acarbon dioxide (CO₂) or a neodymium-doped yttrium aluminum garnet(Nd:YAG) laser source with a wavelength in the infrared region of theelectromagnetic spectrum. For example, the laser source 310 has awavelength of 9.4 μm or 10.6 μm, in an embodiment. The laser light beamLR1 generated by the laser source 300 is guided by the laser guideoptics 320 and focused, by the focusing apparatus 330, into theexcitation laser beam LR2 that is introduced into the EUV radiationsource 100. In some embodiments, in addition to CO₂ and Nd:YAG lasers,the laser beam LR2 is generated by a gas laser including an excimer gasdischarge laser, helium-neon laser, nitrogen laser, transversely excitedatmospheric (TEA) laser, argon ion laser, copper vapor laser, KrF laseror ArF laser; or a solid state laser including Nd:glass laser,ytterbium-doped glasses or ceramics laser, or ruby laser.

In some embodiments, the excitation laser beam LR2 includes a pre-heatlaser pulse and a main laser pulse. In such embodiments, the pre-heatlaser pulse (interchangeably referred to herein as the “pre-pulse) isused to heat (or pre-heat) a given target droplet to create alow-density target plume with multiple smaller droplets, which issubsequently heated (or reheated) by a pulse from the main laser (mainpulse), generating increased emission of EUV light compared to when thepre-heat laser pulse is not used.

In various embodiments, the pre-heat laser pulses have a spot size about100 μm or less, and the main laser pulses have a spot size in a range ofabout 150 μm to about 300 μm. In some embodiments, the pre-heat laserand the main laser pulses have a pulse-duration in the range from about10 ns to about 50 ns, and a pulse-frequency in the range from about 1kHz to about 100 kHz. In various embodiments, the pre-heat laser and themain laser have an average power in the range from about 1 kilowatt (kW)to about 50 kW. The pulse-frequency of the excitation laser beam LR2 ismatched with the ejection-frequency of the target droplets DP in anembodiment.

The laser beam LR2 is directed through windows (or lenses) into the zoneof excitation ZE. The windows adopt a suitable material substantiallytransparent to the laser beams. The generation of the laser pulses issynchronized with the ejection of the target droplets DP through thenozzle 117. As the target droplets move through the excitation zone, thepre-pulses heat the target droplets and transform them into low-densitytarget plumes. A delay between the pre-pulse and the main pulse iscontrolled to allow the target plume to form and to expand to an optimalsize and geometry. In various embodiments, the pre-pulse and the mainpulse have the same pulse-duration and peak power. When the main pulseheats the target plume, a high-temperature plasma is generated. Theplasma emits EUV radiation, which is collected by the collector mirror110. The collector mirror 110, an EUV collector mirror, further reflectsand focuses the EUV radiation for the lithography exposing processesperformed through the exposure device 200.

One method of synchronizing the generation of a pulse (either or both ofthe pre-pulse and the main pulse) from the excitation laser with thearrival of the target droplet in the zone of excitation is to detect thepassage of a target droplet at given position and use it as a signal fortriggering an excitation pulse (or pre-pulse). In this method, if, forexample, the time of passage of the target droplet is denoted by t_(o),the time at which EUV radiation is generated (and detected) is denotedby t_(rad), and the distance between the position at which the passageof the target droplet is detected and a center of the zone of excitationis d, the speed of the target droplet, v_(dp), is calculated as

v _(dp) =d/(t _(rad) −t _(o))  Equation (1).

Because the droplet generator is expected to reproducibly supplydroplets at a fixed speed, once v_(dp) is calculated, the excitationpulse is triggered with a time delay of d/v_(dp) after a target dropletis detected to have passed the given position to ensure that theexcitation pulse arrives at the same time as the target droplet reachesthe center of the zone of excitation. In some embodiments, that thepassage of the target droplet is used to trigger the pre-pulse, the mainpulse is triggered following a fixed delay after the pre-pulse. In someembodiments, the value of target droplet speed v_(dp) is periodicallyrecalculated by periodically measuring trod, if needed, and thegeneration of pulses with the arrival of the target droplets isresynchronized.

In an EUV radiation source 100, the plasma caused by the laserapplication creates debris, such as ions, gases and atoms of thedroplet, as well as the desired EUV radiation. It is necessary toprevent the accumulation of material, e.g., debris, on the collectormirror 110 and also to prevent debris exiting the chamber 105 andentering the exposure device 200.

As shown in FIG. 1 , a buffer gas is supplied from a first buffer gassupply 130 through the aperture in collector mirror 110 by which thepulse laser is delivered to the tin droplets. In some embodiments, thebuffer gas is H₂, He, Ar, N or another inert gas. In certainembodiments, H₂ is used as H radicals that are generated by ionizationof the buffer gas and can be used for cleaning purposes. The buffer gascan also be provided through one or more second buffer gas supplies 135toward the collector mirror 110 and/or around the edges of the collectormirror 110. Further, the chamber 105 includes one or more gas outlets140 so that the buffer gas is exhausted outside the chamber 105.Hydrogen gas has low absorption to the EUV radiation. Hydrogen gasreaching to the coating surface of the collector mirror 110 reactschemically with a metal of the droplet forming a hydride, e.g., metalhydride. When tin (Sn) is used as the droplet, stannane (SnH₄), which isa gaseous byproduct of the EUV generation process, is formed. Thegaseous SnH₄ is then pumped out through the gas outlet 140. However, itis difficult to exhaust all gaseous SnH₄ from the chamber and to preventthe SnH₄ from entering the exposure device 200. Therefore, monitoringand/or control of the debris in the EUV radiation source 100 isbeneficial to the performance of the EUVL system.

FIG. 2 is a schematic view of an EUVL exposure tool in accordance withsome embodiments of the present disclosure. The EUVL exposure tool ofFIG. 2 includes the exposure device 200 that shows the exposure ofphotoresist coated substrate, a target substrate 210, with a patternedbeam of EUV light. The exposure device 200 is an integrated circuitlithography tool such as a stepper, scanner, step and scan system,direct write system, device using a contact and/or proximity mask, etc.,provided with one or more optics 205 a, 205 b, for example, toilluminate a patterning optic 205 c, such as a reticle, with a beam ofEUV light, to produce a patterned beam, and one or more reductionprojection optics 205 d, 205 e, for projecting the patterned beam ontothe target substrate 210. A mechanical assembly (not shown) may beprovided for generating a controlled relative movement between thetarget substrate 210 and patterning optic 205 c. As further shown, theEUVL exposure tool of FIG. 2 , further includes the EUV radiation source100 including a plasma plume 23 at the zone of excitation ZE emittingEUV light in the chamber 105 that is collected and reflected by acollector mirror 110 into the exposure device 200 to irradiate thetarget substrate 210.

FIG. 3 shows a schematic view of plasma formation process throughlaser-metal interaction between a laser beam and a metal droplet inaccordance with some embodiments of the present disclosure. In FIG. 3 ,the ejected metal droplet, e.g., the ejected tin droplet DP, reaches thezone of excitation ZE where it interacts with the laser beam LR2 to forma plasma. The zone of excitation ZE is at a focus of the high-power andhigh-pulse-repetition-rate pulsed laser beam LR2. The laser beam LR2interacts with the ejected tin droplet DP at the ignition site in aspace of the chamber of the EUVL system to form the plasma plume 23which emits EUV light rays 24 in all directions. During this laser-metalinteraction, a tin droplet DP could be missed by or not interactsufficiently with the laser beam LR2, thereby passing to a positionbelow the zone of excitation ZE in FIG. 3 , forming debris droplet 25.Also, some tin leftover from the plasma formation process can becomedebris 26. The debris droplet 25 and debris 26 can accumulate on thesurface of the EUV collector mirror, e.g., collector mirror 110 of FIG.1 , deteriorating the reflective quality of the EUV collector mirror110. The debris 26 and debris droplet 25 contaminate the collectormirrors 110 such that the collector mirror 110 may need to cleanedand/or replaced, thereby increasing the maintenance cost, and moreimportantly, reducing the availability of the EUVL system. Replacing orcleaning the collector mirror 110 is time consuming, for example,replacement of the EUV collector mirror 110 may require up to 4 days.Thus, cleaning or replacing the collector mirror 110 before it is neededincreases the maintenance cost and not cleaning or replacing thecollector mirror 110 when the cleaning or replacement is neededdeteriorates the EUV radiation. Therefore, there is a demand for animproved method of monitoring the debris on collector mirror 110 todetermine when cleaning and/or replacement of the collector mirror 110because of the contamination by the debris droplet 25 and the debris 26is needed. The plasma formation process is described in more detailswith respect to FIG. 4 .

FIG. 4 shows a cross-sectional view of the EUV radiation source in anoperation situation in accordance with some embodiments of the presentdisclosure. The EUV radiation source 100 includes the focusing apparatus330, the collector mirror 110, the target droplet generator 115, anaperture 50 for entering the laser beam LR2, and a drain such as adroplet catcher 85, e.g., a tine catcher, for the unreacted tindroplets, the debris droplet 25. The collector mirror 110 is made of amulti-layered mirror including Mo/Si, La/B, La/B₄C, Ru/B₄C, Mo/B₄C,Al₂O₃/B₄C, W/C, Cr/C, and Cr/Sc with a capping layer including SiO₂, Ru,TiO₂, and ZrO₂, in some embodiments. The diameter of the collectormirror 110 can be about 330 mm to about 750 mm depending on the chambersize of the EUV radiation source 100. The cross-sectional shape of thecollector mirror 110 can be elliptical or parabolic, in someembodiments.

Since the plasma plume 23 includes active and highly charged particlesor ions such as tin (Sn) ions, and a spatial positional error/tolerancemay exist between the tin droplet DP and the focus position of the laserbeam ZE, debris is formed and can be pushed by the high power radiationtoward the lower-half region of the reflective surface of the collectormirror 110, causing contamination of the collector mirror 110. Also, dueto the synchronization control the laser beam pulse frequency and thespeed of the ejected tin droplet DP, some droplets are laser-missed andbecome debris droplets 25 and some droplet under react with the laserbeam. The under-reacted portion of a tin droplet DP may form debris 26which deposits on the lower-half portion of the reflective surface ofthe collector mirror 110. The deposited debris 26 or debris droplets 25deteriorate the reflective property of the collector mirror 110, therebylowering the power of EUV radiation source 100 for EUV photolithographyof the target substrate 210 of FIG. 2 , and lowering the quality (suchas critical dimension CD and line edge roughness ((LER) of patternsformed on the photo-sensitive coating (not shown) on the targetsemiconductor substrate 210. Therefore, there is a demand for monitoringthe debris 26 and debris droplets 25 deposition onto the reflectivesurface of the collector mirror 110.

FIG. 5A shows a schematic view of a collector mirror and relatingportions of an EUV radiation source in accordance with some embodimentsof the present disclosure. FIG. 5A shows a schematic view of the EUVradiation source 100, including a debris collection mechanism 150, thecollector mirror 110, the target droplet generator 115, and the dropletcatcher 85. The circled area 152 in FIG. 5A is shown close up in FIG.5B.

FIG. 5B shows a detailed view of drip holes and a debris receptacle inaccordance with some embodiments of the present disclosure. As shown bythe arrows in FIG. 5B, molten debris, such as excess tin, passes throughdrip holes (or fluid passages) 158 and into a debris receptacle 160(e.g., a tin bucket). The debris receptacle 160 is located outside ofthe optical path of the EUV radiation source 100, in some embodiments.

In some embodiments, the debris receptacle 160 is located behind thecollector mirror 110 of FIG. 5A. In some embodiments, the debrisreceptacle 160 is made of material suitable for collecting moltendebris, such as molten tin. In some embodiments, the debris receptacle160 is made of a steel. The debris receptacle 160 can be cleaned,emptied, or replaced during routine maintenance of the EUV radiationsource, such as when swapping out the collector mirror 110. As shown inFIG. 5B, in some embodiments, there is a plurality of drip holes (orfluid passages) 158 located adjacent the bottom of the debris collectionmechanism 150.

FIG. 6A shows contamination of the EUV collector mirror in the chamberof the EUVL system in accordance with some embodiments of the presentdisclosure. FIG. 6A shows collector mirror contamination of the EUVcollector mirror 110 having the aperture 50. After prolonged use, thearea of the reflective surface of the collector mirror 110 that iscovered by the deposited debris 26 increases and the functioning of thecollector mirror 110 decreases. Without cleaning the contaminatedcollector mirror 110 from the debris 26 or replacing the collectormirror 110, the quality of the pattern formed on the target substrate210 of FIG. 2 using the contaminated collector mirror 110 would bedegraded, affecting the productivity of high quality chips. FIG. 6Bshows an EUV collector mirror after cleaning the surface thereof inaccordance with some embodiments of the present disclosure.

FIGS. 7A and 7B show devices for illuminating and imaging tin dropletsand tin debris in an EUV radiation source in accordance with someembodiments of the present disclosure. FIG. 7A is a plan view 700 of acut of the EUV radiation source 100 of FIG. 1 . FIG. 7A shows a DIM710A, a DDM 720A, the collector mirror 110, and the tin droplets DPmoving from target droplet generator 115 to the zone of excitation ZE.The DIM 710A provides a light beam 740 to illuminate the droplets DP atthe zone of excitation ZE. In some embodiments, the DIM 710A includesone or more light sources, such as a laser source to illuminate the zoneof excitation ZE. In some embodiments, the DDM 720A includes one or moreimage sensors, such as a camera, e.g., a digital camera. In someembodiments, through illuminating the zone of excitation ZE and an areaaround the ZE by the DIM 710A, the camera of the DDM 720A takes at leasttwo images of the area around the zone of excitation ZE after thedroplet DP is hit by laser beam LR2 (not shown). The images are taken,e.g., captured, successively with a slight time difference, e.g., fromabout 200 nano-seconds (ns) to about 200 micro-seconds (ms) between themand thus the images show how the droplets DP moves from one image to thenext image.

In some embodiments, a plurality of DIMs is installed around the EUVradiation source 100. As shown in FIG. 7A, in addition to the DIM 710A,DIMs 710B and 710C are also installed around the EUV radiation source100 such that the light sources of DIMs 710B and 710C illuminatedifferent locations and take different views of the zone of excitationZE. Also, as shown in FIG. 7A, in addition to the DDM 720A, DDMs 720B,720C, 720D, and 720E are also installed around the EUV radiation source100 such that the cameras of the DDMs 720B, 720C, 720D, and 720E takeimages of multiple viewpoints inside the EUV radiation source 100. Insome embodiments, the light source of the DIM 710A provides illuminationin the shape of a light curtain beam 740 having substantially the sameintensity across its profile that illuminates an area, e.g., illuminatesa plane. In some embodiments, the illuminated plane is a first planethat includes the droplet generator 115, the zone of excitation ZE, andthe droplet catcher 85. In some embodiments, the first plane includes across-sectional area 727 between the rims of collector mirror 110. Insome other embodiments, the first plane in addition to thecross-sectional area 727, includes at least a portion of across-sectional area 725 outside the cross-sectional area 727. Thecamera of the DDM 720A takes at least two images of the illuminatedplane. Therefore, the two or images taken by the camera of the DDM 720Ashow the location of the droplets DP that are being released from thedroplet generator 115 before reaching the zone of excitation ZE. In someembodiments, the two or images taken by the camera of the DDM 720A showthe plasma plume 23 of FIG. 3 , and the debris 26 and the debrisdroplets 25 of FIG. 3 that are missed by the laser beam LR2 and dropstowards the droplet catcher 85. As noted, the images are takensuccessively, with a slight time difference between them, and thus theimages show how the droplets DP, the debris droplets 25, and the debris26 move from one image to the next image in the illuminated plane. Insome embodiments, a velocity of the droplets DP, the debris droplets 25,and the debris 26 in the illuminated plane is determined based on thesuccessive images. In some embodiments, the debris 26 of FIG. 3 , doesnot stay in the illuminated plane and thus a velocity of the debris 26is not determined from the successive images of the illuminated plane.In some embodiments, the light curtain beam produced by the DIM 710A orthe light curtain beams produced by the other DIMs 710B or 710C has awidth in the range of about 2000 μm to about 3000 μm.

In some embodiments, the light sources of the DIMs 710A, 710B, and/or710C illuminate multiple parallel planes perpendicular to the firstplane. The parallel planes extend in the volume between the first planeand the collector mirror 110, e.g., an inside surface of the collectormirror 110. In some embodiments, a location of the light sources of theDIMs 710A, 710B, and 710C are controlled by stepper motors such thateach light source moves and provides multiple parallel light curtains,e.g., illuminates multiple parallel planes. In some embodiments, thefirst plane is a vertical plane and the one or more DIMs providemultiple horizontal and vertical illuminated planes in the volumebetween the first plane and the collector mirror 110. The cameras of theDDMs 720A, 720B, 720C, 720D, and 720E, take two or more images, with theslight time difference between consecutive images. Also, the DDMs 720A,720B, 720C, 720D, and 720E, take two or more images from differentviewpoints inside the volume between the first plane and the collectormirror 110. Thus, based on the captured images, a location, size, andvelocity of the debris 26 in the volume between the first plane and thecollector mirror 110 are determined, e.g., sampled. Also, based on thelocation, size, and/or velocity of the debris 26 in the captured images,the flow of the debris 26 can be determined and it is projected todetermine which debris 26 hits the collector mirror. In someembodiments, the amount of debris 26 deposited on the collector mirror110 is calculated and a map of the deposited debris 26 on the collectormirror 110 is generated. As noted, based on the map of the amount ofdebris on the collector mirror, it may be determined when is the timefor the cleaning of the collector mirror or the replacement of thecollector mirror. In some embodiments, when between about 70% to about85% of the collector mirror 110 is covered by the debris, the collectormirror 110 is cleaned.

FIG. 7B illustrate an apparatus for velocimetry of droplets of debris ofan EUV lithography system and monitoring collector mirror contamination,according to some embodiments of the present disclosure. The device 800shows the DIM 710, which is consistent with DIM 710A of FIG. 7A and theDDM 720, which is consistent with the DDM 720A of FIG. 7A. A lightsource of the DIM 710 illuminates the tin droplets DP, the debrisdroplets 25, and the debris 26 in the EUV radiation source 100. Thedevice 800 further captures images of the tin droplets DP, the debrisdroplets 25, and the debris 26 in the EUV radiation source 100.

In an embodiment, the light source of the DIM 710 is used forilluminating, by light beam 740, the zone of excitation ZE and aroundthe zone of excitation ZE that includes a target droplet DP ejected byfrom the nozzle 117 of the droplet generator 115 and moving in adirection 810, e.g., a vertical direction. As discussed, in someembodiments, the light beam 740 is a light curtain beam that illuminatesa plane that includes the zone of excitation ZE, which also includes oneor more of the tin droplets DP, the debris droplets 25, and the debris26. The reflected or scattered light 820 from the target droplet DP andthe debris droplets 25, the reflected or scattered light 820 from thedebris 26, and/or the reflected or scattered light 820 from debris inthe plasma plume 23 is captured by an image sensor, e.g., a camera, inthe DDM 720. In some embodiments and consistent with FIG. 7A, one ormore other DIMs, having corresponding light sources, are included in thedevice 800 and the other DIMs are used to illuminate other parts and/orother views of the EUV radiation source 100. Also, in some embodimentsand consistent with FIG. 7A, one or more other DDMs having correspondingimage sensors, e.g., cameras, are included in the device 800 and theother camera are used for capturing the reflected or scattered lightfrom the target droplet DP, the debris droplets 25, and the debris 26.The use of additional light beams 740 of the light sources of the otherDIMs and using the cameras of the other DDMs allows capturing imagesfrom multiple locations and viewpoints inside the EUV radiation source100. As noted above, the camera of DDM 720 and the cameras of the otherDDMs, take two or more images, with the slight time difference betweenconsecutive images. Thus, the device 800 is used for velocimetry bycalculating e.g., determining, a velocity of the target droplet DP, thedebris droplets 25, and the debris 26 of the entire inner space of theEUV radiation source 100. The velocity is determined by analyzing thecaptured consecutive images of each viewpoint as will be described withrespect to FIG. 9B.

In some embodiments, when the laser beam LR2, the excitation laser beam,hits the target droplet DP within the zone of excitation ZE, the plasmaplume 23 forms because of ionization of the target droplet DP thatcauses the target droplet DP to expand rapidly into a volume. The volumeof the plasma plume 23 dependents on the size of the target droplet DPand the energy provided by the laser beam LR2. In various embodiments,the plasma expands several hundred microns from the zone of excitationZE. As used herein, the term “expansion volume” refers to a volume towhich plasma expands after the target droplets are heated with theexcitation laser beam LR2.

In some embodiments, the DIM 710 includes a continuous wave laser. Inother embodiments, the DIM 710 includes a pulsed laser. The wavelengthof the laser of the DIM 710 is not particularly limited. In someembodiments, the laser of the DIM 710 has a wavelength in the visibleregion of electromagnetic spectrum. In some embodiments, the DIM 710 hasa wavelength of about 1070 nm. In some embodiments, the laser of the DIM710 has an average power in the range from about 1 W to about 50 W. Forexample, in some embodiments, the laser of the DIM 710 has an averagepower of about 1 W, about 5 W, about 10 W, about 25 W, about 40 W, about50 W, or any average power between these values. In some embodiments,the DIM 710 generates a beam having a uniform illumination profile. Forexample, in some embodiments, the DIM 710 creates a fan-shaped lightcurtain or a thin plane of light having substantially the same intensityacross its profile.

As the target droplet DP passes through the beam generated by the DIM710, the target droplet DP reflects and/or scatters the photons in thebeam. In an embodiment, the target droplet DP produces a substantiallyGaussian intensity profile of scattered photons. The photons scatteredby the target droplet DP are detected by the DDM 720. In someembodiments, the peak of the intensity profile detected by the DDM 720corresponds to the center of the target droplet DP. In some embodiments,the DDM 720 includes a photodiode and generates an electrical signalupon detecting the photons reflected and/or scattered by the targetdroplet DP. In some embodiments, the DDM 720 includes a camera andgenerates two or more consecutive images upon of the photons reflectedand/or scattered by the target droplet DP.

In an embodiment, a synchronizer 730 synchronizes the illumination lightbeam 740 generated by the DIM 710 with the recording of the illuminationlight reflected from or scattered by the particles to the DDM 720. Insome embodiments, a controller 750 controls and synchronizes the DIM710, the DDM 720, the synchronizer 730, the releasing of tin droplets DPby the droplet generator 115. In addition, the controller 750 provides atrigger signal to the laser source 300 of FIG. 1 that generates thelaser beam LR2 such that a laser pulse generating the laser beam LR2 issynchronized with the releasing of tin droplets DP, the DIM 710, and theDDM 720. In some embodiments, the controller 750 controls the DIM 710and DDM 720 through the synchronizer 730. In some embodiments, thesynchronizer 730 does not exist and the controller 750 directly controlsand synchronizes the DIM 710, the DDM 720, the laser source 300, and thedroplet generator 115.

In some embodiments, particle image velocimetry is used to monitor theflow of one or more of debris 26, plasma plume 23, and gases such ashydrogen, in the EUV radiation source 100. Particle image velocimetry(PIV) is an optical method of flow visualization used to obtaininstantaneous velocity measurements and related properties in fluids.Tracer particles that are sufficiently small enough to follow the flowdynamics are illuminated so that particles are visible. The particlesare imaged and the motion of the tracer particles is used to calculatespeed and direction (the velocity) of the flow of the fluid. In someembodiments, the tracer particles are particles of debris 26 forvelocimetry of the gases, e.g., the hydrogen, in the EUV radiationsource 100. In some embodiments, the velocimetry of metal particles (tinparticles) is performed to determine a flow of tin particles and tocalculate how much tin particles is deposited on the collector mirror.

PIV produces two-dimensional or even three-dimensional vector fields.During PIV, the particle concentration is such that it is possible toidentify individual particles in an image, but not with certainty totrack it between images. When the particle concentration is so low thatit is possible to follow an individual particle, it is called ParticleTracking Velocimetry, while Laser Speckle Velocimetry is used for caseswhere the particle concentration is so high that it is difficult toobserve individual particles in an image.

In some embodiments, the PIV apparatus includes a droplet detectionmodule (DDM) 720, such as a digital camera with a CCD chip, a dropletillumination module (DIM) 710, such as a strobe or laser with an opticalarrangement to limit the physical region illuminated. In someembodiments, the DIM 710 includes a cylindrical lens to convert a lightbeam to a line. In some embodiments, the PIV includes a synchronizer 730to act as an external trigger for control of the camera and illuminationlight source. In some embodiments, a fiber optic cable or liquid lightguide connect the illumination light source to the lens setup. Thecontroller 950 is programmed with PIV software to post-process theoptical images.

To perform PIV analysis on the flow, two exposures of the illuminationlight are required upon the DDM from the flow. Digital cameras using CCDor CMOS image sensors can capture two frames at high speed with a fewhundred ns difference between the frames. This enables each exposure tobe isolated on its own frame for accurate cross-correlation analysis.

In some embodiments of the PIV apparatus, lasers are used as the DIM 710due to their ability to produce high-power light beams with short pulsedurations. This yields short exposure times for each frame. In someembodiments, Nd:YAG lasers are used in PIV setups. The Nd:YAG lasersemit primarily at the 1064 nm wavelength and its harmonics (532, 266,etc.). For safety reasons, the laser emission is typically bandpassfiltered to isolate the 532 nm harmonics (this is green light, the onlyharmonic able to be seen by the naked eye).

The optics include a spherical lens and cylindrical lens combination insome embodiments. The cylindrical lens expands the laser into a planewhile the spherical lens compresses the plane into a thin sheet. Itshould be noted though that the spherical lens cannot compress the lasersheet into an actual 2-dimensional plane. The minimum thickness is onthe order of the wavelength of the laser light and occurs at a finitedistance from the optics setup (the focal point of the spherical lens).The lens for the camera should also be selected to properly focus on andvisualize the particles within the investigation area.

The synchronizer 730 acts as an external trigger for both the DDM 720and the DIM 710. The controller 750 controls the synchronizer 730, DIM710, and DDM 720. The synchronizer 730 can dictate the timing of eachframe of the DIM sequence in conjunction with the firing of theillumination light source to within 1 ns precision. Thus, the timebetween each pulse of the laser and the placement of the laser shot inreference to the camera's timing can be accurately controlled. Knowledgeof this timing is critical as it is needed to determine the velocity ofthe fluid in the PIV analysis. Stand-alone electronic synchronizers,called digital delay generators, offer variable resolution timing fromas low as 250 ps to as high as several milliseconds. With up to eightchannels of synchronized timing, they offer the means to control severalflash lamps and Q-switches as well as provide for multiple cameraexposures.

The frames are split into a large number of interrogation areas, orwindows, in some embodiments. It is then possible to calculate adisplacement vector for each window with help of signal processing andautocorrelation or cross-correlation techniques. This is converted to avelocity using the time between laser shots and the physical size ofeach pixel on the camera. The size of the interrogation window in someembodiments is selected to have at least 6 particles per window onaverage. The synchronizer 730 controls the timing between imageexposures and also permits image pairs to be acquired at various timesalong the flow. The scattered light from each particle is in the regionof 2 to 4 pixels across on the image in some embodiments. If too largean area is recorded, particle image size drops and peak locking mightoccur with loss of sub pixel precision.

FIG. 8A schematically illustrates an apparatus for measuring a velocityof the target droplet DP, the debris droplets 25, or the debris 26 inthe EUV radiation source 100, in accordance with some embodiments of thepresent disclosure. In an embodiment, the apparatus includes the DIM710, the DDM 720, a controller 950 and a processor 900.

In some embodiments, the DIM 710 includes a radiation source 915, a tiltcontrol mechanism 913 and a slit control mechanism 917. The tilt controlmechanism 913 (also referred to herein as “auto tilt”) controls the tiltof the radiation source 915, which is consistent with the EUV radiationsource 100 of FIG. 1 . In some embodiments, the auto tilt 913 is astepper motor coupled to the radiation source 915 (e.g., a laser source)of the DIM 710 and moves the radiation source 915 to change the angle ofincidence at which light (or radiation) L is incident on the targetdroplet DP or plasma plume 23 (and in effect changing the amount oflight R reflected and/or scattered by the target droplet DP or plasmaplume 23 into the DDM 720). In some embodiments, the auto tilt 913includes a piezoelectric actuator. In some embodiments, the light R isalso reflected from the debris droplet 25 and the debris 26. In someembodiments, the illumination system 920 receives light beam LO, e.g., alaser beam, from radiation source 915 and transforms the light beam LOinto light beam L, which is a thin plane of light (a light curtain).

The slit control mechanism 917 (also referred to herein as “auto slit”)controls the amount of light that illuminates the zone of excitation ZE.In some embodiments, an illumination system 920 is disposed between theradiation source 915 and the zone of excitation ZE. The slit controlmechanism 917 of the illumination system 920 controls the amount oflight which irradiates the target droplet DP, the plasma plume 23, thedebris droplets 25, and the debris 26. In some embodiments, theillumination system 920 includes a movable opaque barrier 914, asdepicted in FIG. 8B, having several slits (narrow openings) 914 a, 914b, 914 c of different sizes and the slit control mechanism 917determines which slit the light beam passes through. When, for example,the controller 950 determines that the intensity of light detected atthe DDM 720 is lower than the acceptable range, the controller 950commends the slit control mechanism 917 to move the slits such that awider slit 914 a is provided in the path of light in the illuminationsystem 920, allowing more light to irradiate the zone of excitation ZE,increasing the detected intensity. On the other hand, if it isdetermined that the intensity of light detected at the DDM 720 is higherthan the acceptable range, the controller 950 commands the slit controlmechanism 917 to move the slits such that a narrower slit 914 c isprovided in the path of light in the illumination system 920, therebyreducing the detected intensity. In some embodiments, the parameters ofthe DIM 710 adjusted by the controller 950 includes the width of theslit in the opaque barrier 914 in the path of light beam L exiting theillumination system 920.

While the auto tilt 913 and auto slit 917 are depicted in the FIG. 8A asbeing separate from the radiation source 915, in some embodiments, theauto tilt 913 and the auto slit 917 can be integrated with the radiationsource 915 to form a single DIM 710. In such embodiments, the couplingbetween the controller 950 and the DIM 710 can be suitably modified toprovide the same result as disclosed herein. The controller 950, thus,sets the intensity of light detected at the DDM 720 to enable a stabledetection of target droplets over a duration of time.

FIG. 9A is an exemplary graph 910 of the gas flow in an extremeultraviolet radiation source according to embodiments of the presentdisclosure. The graph 910 shows a cross-sectional view of the EUVradiation source 100 that includes the collector mirror 110, the dropletgenerator 115, the droplet catcher 85, and heat shields 925. In someembodiments and consistent with FIG. 1 , the laser beam LR2 enters fromthe opening 930 of the collector mirror 110. The arrows of the graph 910show the gas flow, e.g., the hydrogen flow, inside the EUV radiationsource 100. As discussed with respect to FIG. 3 , the laser beam LR2 mayinteract with a droplet DP released from the droplet generator 115 toform the laser plume 23 that emits the EUV light rays 24 in alldirections. During the laser-metal interaction, a droplet DP could bemissed, thereby forming debris droplet 25. Also, some tin leftover fromthe plasma formation process can become debris 26. In some embodiments,the particles, e.g., the debris droplet 25 and debris 26, areinfluenced, e.g., carried, by the gas flow. Because the particles havemuch higher mass than the gas, the gas flow although influences the flowof the particles but the particle do not follow the gas flow. Asdescribed above the particle flow may be determined using the PIVanalysis. As shown in the graph 910, the gas flow near the inner surfaceof the collector mirror 110 is tangential to reduce the deposition ofthe debris 26 on the collector mirror 110.

FIG. 9B is an exemplary graph 955 of the particle flow in an extremeultraviolet radiation source according to embodiments of the presentdisclosure. In some embodiments, the graph 955 shows the particle flowin a portion of the inside of the EUV radiation source 100. In FIG. 9B,the dots 905, are metal particles, e.g., tin particles such as debris26. The direction and size of the arrows 907 show the velocity of thedots 905 (e.g., metal particles) that are determined using the PIVanalysis. Thus, the combination of the arrows 907 show a flow of themetal particles. In some embodiments and returning back to FIG. 8A, theprocessor 900 receives the consecutive images that are captured by acamera of DDM 720 and by applying image processing methods such as blobanalysis determines the metal particle inside each image. The processor900 further applies image processing methods, e.g., segmentationmethods, to divide the images into multiple regions and further appliesimage processing methods such as correlation to compare the regions ofconsecutive images to determine the same metal particles in consecutiveimages. The processor further determines the velocity of a metalparticle based on the location of the same particles in the consecutiveimages and a time difference between the consecutive images.

FIG. 10 illustrates a flow diagram of an exemplary process 1000 forvelocimetry of droplets of an EUV lithography system according to someembodiments of the disclosure. In operation S1010, a target droplet,e.g., debris droplet 25 of FIG. 3 , is irradiated in an extremeultraviolet radiation source 100 (light source) of an extremeultraviolet lithography tool with light from a droplet illuminationmodule, e.g., DIM 710 of FIG. 7B. The light reflected and/or scatteredby the target droplet is detected, e.g., by DDM 720, in operation S1020.Next, particle image velocimetry is performed, e.g., by controller 950of FIG. 8A, in operation S1030 to monitor one or more flow parametersinside the extreme ultraviolet radiation source. In some embodiments,one or more operating parameters of the extreme ultraviolet radiationsource are adjusted based on the monitored flow parameters in operationS1040.

FIGS. 11A and 11B illustrate an apparatus for velocimetry of droplets ofdebris of an EUV lithography system and monitoring collector mirrorcontamination, according to some embodiments of the present disclosure.FIG. 11A is a schematic view of a computer system that performs thevelocimetry of droplets of debris in an EUV lithography system. All ofor a part of the processes, method and/or operations of the foregoingembodiments can be realized using computer hardware and computerprograms executed thereon. The operations includes determining an amountof debris deposited on the collector mirror. In FIG. 11A, a computersystem 1100 is provided with a computer 1101 including an optical diskread only memory (e.g., CD-ROM or DVD-ROM) drive 1105 and a magneticdisk drive 1106, a keyboard 1102, a mouse 1103, and a monitor 1104.

FIG. 11B is a diagram showing an internal configuration of the computersystem 1100. In FIG. 11B, the computer 1101 is provided with, inaddition to the optical disk drive 1105 and the magnetic disk drive1106, one or more processors, such as a micro processing unit (MPU), aROM 1112 in which a program such as a boot up program is stored, arandom access memory (RAM) 1113 that is connected to the MPU 1111 and inwhich a command of an application program is temporarily stored and atemporary storage area is provided, a hard disk 1114 in which anapplication program, a system program, and data are stored, and a bus1115 that connects the MPU 1111, the ROM 1112, and the like. Note thatthe computer 1101 may include a network card (not shown) for providing aconnection to a LAN.

The program for causing the computer system 1100 to execute thefunctions of an apparatus for performing the velocimetry of droplets ofdebris and monitoring collector mirror contamination in the foregoingembodiments may be stored in an optical disk 1121 or a magnetic disk1122, which are inserted into the optical disk drive 1105 or themagnetic disk drive 1106, and transmitted to the hard disk 1114.Alternatively, the program may be transmitted via a network (not shown)to the computer 1101 and stored in the hard disk 1114. At the time ofexecution, the program is loaded into the RAM 1113. The program may beloaded from the optical disk 1121 or the magnetic disk 1122, or directlyfrom a network. The program does not necessarily have to include, forexample, an operating system (OS) or a third party program to cause thecomputer 1101 to execute the functions of the photo mask data generatingand merging apparatus in the foregoing embodiments. The program may onlyinclude a command portion to call an appropriate function (module) in acontrolled mode and obtain desired results.

In some embodiments and returning back to FIG. 7B or 8A, the DDM 720includes a photodiode designed to detect light having a wavelength ofthe light from the DIM 710. In some embodiments, the DDM 720 furtherincludes one or more filters for filtering certain frequencies of light.For example, in an embodiment, the DDM 720 includes a filter forblocking deep ultraviolet (DUV) radiation. In another embodiment, theDDM 720 includes a filter for blocking all frequencies other than thatof the light from the DIM 710.

Referring back to FIG. 7B, in some embodiments, the controller 750 is alogic circuit programmed to receive a signal from the DDM 720, anddepending on the received signal transmit control signals to one or morecomponents of the DIM 710 to automatically adjust one or more operatingparameters of the EUV radiation source.

In some embodiments, as shown in FIG. 7A, two or more light sources,e.g., lasers, and/or two or more image sensors, e.g., cameras areprovided in the EUV radiation source to monitor the flows in the entireinner space of the EUV radiation source.

In the present disclosure, by performing particle image velocimetry, thetiming of tin droplet generation and irradiation is improved in someembodiments. In some embodiments, a plasma flow is optimized as a resultof the particle image velocimetry. In some embodiments, based onparticle image velocimetry the operating parameters of the EUV radiationsource are adjusted to optimize the debris field and to limit thedeposition of debris on the surface of the collector mirror. Forexample, the flow of the gasses (e.g., hydrogen flow) in the EUVradiation source 100 is increased to reduce the contamination of thecollector mirror 110. In some embodiments, a position where the laserbeam LR2 hits the droplet DP is adjusted and/or a time of the laserpulse generating the laser beam LR2 is adjusted to reduce thecontamination of the collector mirror 110.

According to some embodiments of the present disclosure, a method formonitoring flow parameters includes irradiating a target droplet in anextreme ultraviolet (EUV) light source of an extreme ultravioletlithography tool with non-ionizing light from a droplet illuminationmodule. The method also includes detecting light reflected and/orscattered by the target droplet. The method further includes performingparticle image velocimetry, based on the detected light, to monitor oneor more flow parameters inside the EUV light source. In an embodiment,the method further includes adjusting one or more operating parametersof the EUV light source based on the monitored flow parameters. In anembodiment, the monitored flow parameters include one or more of a flowpattern of gases, droplets, or debris in the EUV light source, thedroplets and debris propagation direction, and spatial evolution of aplasma shockwave. In an embodiment, the method also includes monitoringa rate of an amount of droplet and debris depositing on a collectormirror of the EUV light source and adjusting the one or more operatingparameters of the EUV light source to reduce the rate. In an embodiment,the method further includes mapping the amount of droplet and debrisdeposited on the collector mirror and triggering a cleaning mechanism toclean the collector mirror based on the mapping. In an embodiment, thenon-ionizing light irradiating the target droplet has a wavelength ofabout 1064 nm. In an embodiment, the source of the non-ionizing light ofthe droplet illumination module is a laser. In an embodiment, the lightreflected and/or scattered by the target droplet is detected by adroplet detection module. In an embodiment, the droplet detection modulecomprises a digital camera.

According to some embodiments of the present disclosure, a method formonitoring a rate of deposition of metal debris includes irradiating oneor more of tin droplets and tin debris in an extreme ultraviolet lightsource of an extreme ultraviolet lithography tool with non-ionizinglight from a droplet illumination module. The method includes detectinglight reflected and/or scattered by the one or more of the tin dropletsand the tin debris. The method also includes performing particle imagevelocimetry, based on the detected light, to monitor a rate of an amountof the tin droplets and the tin debris depositing on a collector mirrorof the extreme ultraviolet light source. In an embodiment, the methodfurther includes adjusting one or more operating parameters of theextreme ultraviolet light source to reduce the rate. In an embodiment,the method further includes mapping the amount of tin droplets and tindebris deposited on the collector mirror and triggering a replacementmechanism to change the collector mirror based on the mapping. In anembodiment, the method further includes mapping the amount of tindroplets and tin debris deposited on the collector mirror anddetermining a half life time of the collector minor based on themapping.

According to some embodiments of the present disclosure, a method forreducing a rate of deposition of metal debris on a collector mirrorincludes irradiating one or more metal debris in an extreme ultravioletlight source of an extreme ultraviolet lithography tool withnon-ionizing light. The method includes detecting light reflected and/orscattered by the one or more metal debris. The method also includesperforming particle image velocimetry, based on the detected light, tomonitor a rate of an amount of the metal debris depositing on thecollector mirror of the extreme ultraviolet light source. The methodfurther includes adjusting one or more flow parameters of gases in theextreme ultraviolet light source to reduce the rate of deposition of themetal debris.

According to some embodiments of the present disclosure, an apparatusfor monitoring flow parameters of particles in an extreme ultravioletlight source of an extreme ultraviolet lithography system includes adroplet illumination module that includes a radiation source forilluminating a target droplet. The apparatus also includes a dropletdetection module for detecting light reflected and/or scattered by thetarget droplet and a controller coupled to the droplet illuminationmodule and the droplet detection module. The droplet detection moduleperforms particle image velocimetry to monitor one or more flowparameters inside the extreme ultraviolet light source. In anembodiment, the controller is further programmed to adjust one or moreoperating parameters of the extreme ultraviolet light source based onthe monitored flow parameters. In an embodiment, the radiation sourcecomprises a laser. In an embodiment, the laser produces a non-ionizinglight having a wavelength of about 1064 nm. In an embodiment, theapparatus further includes a synchronizer that synchronizes the dropletillumination module and the droplet detection module. In an embodiment,the controller also controls the synchronizer.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method used with an extreme ultraviolet (EUV)light source, comprising: irradiating a target droplet with anexcitation laser at a zone of excitation inside the EUV light source;irradiating an area including the zone of excitation with non-ionizinglight; detecting non-ionizing light reflected and/or scattered from thetarget droplet and a debris; and generating a particle image velocimetryfor at least one of the target droplet and the debris based on thedetected non-ionizing light.
 2. The method of claim 1, furthercomprising adjusting one or more operating parameters of the EUV lightsource based on the particle image velocimetry.
 3. The method of claim2, wherein the particle image velocimetry comprises a flow pattern ofgases, a flow pattern of droplets, a flow pattern of the debris in theEUV light source, a droplet or debris propagation direction, or aparameter of a plasma shockwave.
 4. The method of claim 3, furthercomprising: monitoring a rate of an amount of droplet and debrisdepositing on a collector mirror of the EUV light source; and adjustingthe one or more operating parameters of the EUV light source to reducethe rate.
 5. The method of claim 4, further comprising: mapping theamount of droplet and debris deposited on the collector mirror; andtriggering a cleaning mechanism to clean the collector mirror based onthe mapping.
 6. The method of claim 4, further comprising: mapping theamount of droplet and debris deposited on the collector mirror; andtriggering a replacement mechanism to change the collector mirror basedon the mapping.
 7. The method of claim 1, wherein the non-ionizing lightirradiating the target droplet has a wavelength of about 1064 nm.
 8. Themethod of claim 1, wherein a source of the non-ionizing light is alaser.
 9. The method of claim 1, wherein the non-ionizing lightreflected and/or scattered by the target droplet is detected by adroplet detection module.
 10. The method of claim 9, wherein the dropletdetection module comprises a digital camera.
 11. A method used with anextreme ultraviolet (EUV) light source, comprising: irradiating a tindroplet with an excitation laser at a zone of excitation inside the EUVlight source; irradiating an area including the zone of excitation withnon-ionizing light; detecting non-ionizing light reflected and/orscattered from one or more tin droplets and tin debris; and generatingparticle image velocimetry, based on the detected non-ionizing light, tomonitor a rate of an amount of the tin droplets and the tin debrisdepositing on a collector mirror of the EUV light source.
 12. The methodof claim 11, further comprising: adjusting one or more operatingparameters of the extreme ultraviolet light source to reduce the rate.13. The method of claim 11, further comprising: mapping an amount of tindroplets and tin debris deposited on the collector mirror; andtriggering a replacement mechanism to change the collector mirror basedon the mapping.
 14. The method of claim 11, further comprising: mappingan amount of tin droplets and tin debris deposited on the collectormirror; and determining a half lifetime of the collector mirror based onthe mapping.
 15. A method used with an extreme ultraviolet (EUV) lightsource, comprising: detecting non-ionizing light reflected and/orscattered from a target droplet in an extreme ultraviolet (EUV) lightsource of an extreme ultraviolet lithography tool; and determining avelocity of the target droplet, based on the detected non-ionizinglight, to determine a flow parameter inside the EUV light source. 16.The method of claim 15, further comprising adjusting one or moreoperating parameters of the EUV light source based on the determinedflow parameter.
 17. The method of claim 16, wherein the determined flowparameter includes a flow pattern of gases, a flow pattern of droplets,a flow pattern of the debris in the EUV light source, a droplet ordebris propagation direction, or a parameter of a plasma shockwave. 18.The method of claim 17, further comprising: monitoring a rate of anamount of droplet and debris depositing on a collector mirror of the EUVlight source; and adjusting the one or more operating parameters of theEUV light source to reduce the rate.
 19. The method of claim 18, furthercomprising: mapping the amount of droplet and debris deposited on thecollector mirror; and triggering a cleaning mechanism to clean thecollector mirror based on the mapping.
 20. The method of claim 18,further comprising: mapping the amount of droplet and debris depositedon the collector mirror; and triggering a replacement mechanism tochange the collector mirror based on the mapping.